Die Assembly And Method Of Extruding Cellular Ceramic Substrates With A Skin

ABSTRACT

An extrusion apparatus including a die and a mask are provided such that no slots feed directly into the longitudinal skin forming gap between the mask and the die. In a method of forming a die adapted to improve skin uniformity of extruded cellular ceramic substrates a slotted block of die material is provided including central slots adapted to form a cellular matrix of the substrate and peripheral slots located outwardly of the central slots designed to be covered by a skin former mask and adapted to extrude peripheral batch material. An arcuate skin former is cut corresponding to a target shrinkage so as to intersect the slotted block such that skin flow from tangent slots at 90 degree positions of the die is limited to the peripheral batch material.

CROSS RELATED TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/929,055, filed on Jun. 27, 2013, which is a divisional of U.S. patentapplication Ser. No. 12/786,983, filed on May 25, 2010, which claimspriority from and the benefit of U.S. Provisional Application No.61/181,817, filed on May 28, 2009, all of which are hereby incorporatedby reference for all purposes as if fully set forth herein.

FIELD

The present disclosure is directed to dies adapted to extrude cellularsubstrates from plasticized ceramic-forming batch materials and, inparticular, honeycomb substrate with extruded skin.

BACKGROUND

Skinned honeycomb extrusion is accomplished by extruding plasticizedceramic-forming batch materials, such as cordierite ceramic formingbatch materials, through honeycomb extrusion dies to form structureshaving a central webbed cellular honeycomb structure surrounded by athin integral outer skin layer. Such skins provide additional strengthto such honeycomb articles. Typically, the honeycomb extrusion diesemployed to produce such skinned honeycomb articles are multi-componentassemblies including, for example, a web-forming die body combined witha skin-forming mask. U.S. Pat. Nos. 4,349,329 and 4,298,328 exemplifydie structures including skin-forming masks. The die body typicallyincludes batch feed holes leading to and intersecting with an array ofthin discharge slots formed in the die face, through which the batchmaterial is extruded. This extrusion forms an interconnecting array ofcrisscrossing thin webs forming the central cellular honeycombstructure. The mask is generally a ring-like circumferential structure,typically in the form of a collar, defining the periphery of the skin ofthe honeycomb. The circumferential skin layer of the honeycomb articleis formed by extruding the batch material between the mask and the diebody.

SUMMARY

In one aspect, an extrusion apparatus is disclosed herein comprising: anextrusion die having an upstream side and a downstream side disposedlongitudinally opposite from the upstream side, the die comprising aplurality of spaced apart pins defining an interconnected matrix oflongitudinal slots opening to the downstream side, the pins havingrespective generally co-planar tops forming, at the upstream side, adischarge surface and a recessed skin former surface disposed radiallyoutwardly of the discharge surface, the pins being comprised of boundarypins having tops terminating at the discharge surface and being disposedat the outer periphery of the discharge surface, and interior pins beingdisposed radially inward of the boundary pins; and an annular maskcomprising an upstream transverse wall and an inward facing longitudinalwall; wherein the upstream transverse wall of the annular mask is spacedaway from the recessed skin former surface in a longitudinal direction,thereby longitudinally masking the longitudinal slots terminating in therecessed skin former surface, to provide a skin former reservoir betweenthe mask and the die; wherein the inward facing longitudinal wall of themask is radially spaced away from the tops of the pins to form a facegap F that provides a terminal opening for the skin former reservoirproximate the tops of the pins; and wherein the mask longitudinallymasks the slots terminating at the skin former surface at a locationimmediately adjacent the die where at least one of the longitudinalslots founed by one or more boundary pins and the corresponding one ormore immediately adjacent interior pins is disposed substantiallyperpendicularly to the inward facing longitudinal wall.

In another aspect, a method is disclosed herein for forming a dieadapted to improve skin uniformity and thus strength of extruded ceramicsubstrates. Extrusion dies are designed to match the batch shrinkage ofthe ceramic composition from which the substrate will be formed. Smallchanges in shrinkage can be managed in the manufacturing process toobtain exact contour for specific customers. Utilizing the adjustabilityof the shrinkage it is possible to design dies to particular targetshrinkage ranges that permit greater control of skin uniformity of thesubstrate than might be the case at other particular ranges ofshrinkage. This greater control is gained by choosing target shrinkagesand corresponding die assembly parameters that maintain pin integrityand control slot location at 90 degree positions of the die.

In some embodiments, a method of forming a die adapted to improve skinuniformity of extruded cellular ceramic substrates, comprising providinga slotted block of die material including central slots adapted to forma cellular matrix of the substrate and peripheral slots locatedoutwardly of the central slots designed to be covered by a skin formermask and adapted to extrude peripheral batch material. A targetshrinkage is selected. An arcuate skin former is cut corresponding tothe target shrinkage so as to intersect the slotted block such that skinflow from tangent slots at 90 degree positions of the die is limited tothe peripheral batch material. (i.e., fed only from tangential “outerslots” at the 90's as defined below). By limiting skin flow at the 90'sto peripheral batch material fed from the outer slots, no tangent slotfeeds directly into the face gap at the 90's, which leads to enhancedskin uniformity.

In some embodiments, values for slot width (W), slot spacing (S) andface gap (F) are determined, and ranges of values for inner slot to pinface (I) and outer slot to mask (O) are obtained, according to thefollowing Equations I:

Omin=a minimum outer slot to mask distance=½W;

Imax=a maximum inner slot to pin face distance=S−F−Omin;

Imin=a minimum inner slot to pin face distance=½W; and

Omax=a maximum outer slot to mask distance=S−F−Imin,

wherein the skin former is cut at O and I values within the ranges.

In other embodiments, first values for O and I can be calculated using afirst shrinkage, outer dimension of fired substrate, mask radius and pinface radius; and it can be determined whether the first values for O andI fall within the ranges for O and I (according to Equations I). If thefirst values for O and I fall within the ranges for O and I, then theskin former is cut at the first values of O and I corresponding to thefirst target shrinkage. If the first values for O and I do not fallwithin the ranges for O and I, then second values for O and I in therange are selected, and the skin former is cut at the second values forO and I corresponding to the second target shrinkage.

A difference in skin thickness corresponding to the 90 degree positionsof the die and the 45 degree positions of the die is not more than 3.00mils (one mil being 1×10⁻³ inch). This can also result in firedsubstrates having an average skin thickness of not more than 15 mils.

Many additional features, advantages and a fuller understanding of theinvention will be had from the accompanying drawings and the detaileddescription that follows. It should be understood that the above Summaryof the Invention describes the invention in broad terms while thefollowing Detailed Description describes the invention more narrowly andpresents embodiments that should not be construed as necessarylimitations of the broad invention as defined in the claims.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 is a top view of a die constructed in accordance with the presentinvention without a skin former mask in position.

FIG. 2 is a vertical cross-sectional view of a portion of the die ofFIG. 1 with the skin former mask.

FIG. 3 is a top plan view taken from the dotted lines labeled “FIG. 3”in FIG. 1.

FIG. 4 is a cross-sectional view of a portion of a die and skin formermask taken at a 90 degree position of the die (designed for 4.0%shrinkage) as seen from the plane labeled 4-4 in FIG. 3.

FIG. 5 is a perspective view of the portion of the die shown in FIG. 3.

FIG. 6 is a photograph of a 90 degree portion of a fired ceramicsubstrate produced by the die shown in FIGS. 3-5.

FIG. 7 is a cross-sectional view taken at the 90 degree position of adie (designed for 4.4% shrinkage) that was not constructed according tothe invention.

FIG. 8 is a top plan view of a portion of another die that was notconstructed in accordance with the invention.

FIG. 9 is a cross-sectional view taken at a 90 degree position of thedie (designed for 5% shrinkage) as seen from the plane labeled 9-9 inFIG. 8.

FIG. 10 is a perspective view of the portion of the die shown in FIG. 8.

FIG. 11 is a photograph of a 90 degree portion of a fired ceramicsubstrate produced by the die shown in FIGS. 8-10).

DETAILED DESCRIPTION

Thermal durability of cellular ceramic substrates has been correlated to“skin thinness” and uniformity. Substrates with thicker skin and lessuniform skin thickness fail at lower temperatures in thermal shocktesting. Thinner skin (e.g., three or four times the web thickness)creates a more thermally resistant design. There are many hardwareconfigurations and die designs utilized in the extrusion process tofacilitate forming “thin” skin, including reduced skin former depth,multiple step skin former cut (step burn), face gap and shim design.U.S. Pat. Nos. 4,668,176 and 4,710,123, for example, describe diedesigns wherein skin thickness can be controlled by adjusting the widthof the gap between the die body and mask. Also shown are means foradjusting the supply of batch material to the skin-forming region of thedie. Skin forming adjustments are currently made, for example, usingmasks, shims and fiow-plates placed in front of or behind the die as itis used to extrude ceramic substrates. These tools control bulk flow andvelocity of the material and are primarily concerned with controllingthe outer diameter of the skin.

In one aspect, an extrusion apparatus is disclosed herein comprising: anextrusion die having an upstream side and a downstream side disposedlongitudinally opposite from the upstream side, the die comprising aplurality of spaced apart pins defining an interconnected matrix oflongitudinal slots opening to the downstream side, the pins havingrespective generally co-planar tops forming, at the upstream side, adischarge surface and a recessed skin former surface disposed radiallyoutwardly of the discharge surface, the pins being comprised of boundarypins having tops terminating at the discharge surface and being disposedat the outer periphery of the discharge surface, and interior pins beingdisposed radially inward of the boundary pins; an annular maskcomprising an upstream transverse wall and an inward facing longitudinalwall, wherein the upstream transverse wall of the annular mask is spacedaway from the recessed skin former surface in a longitudinal direction,thereby longitudinally masking the longitudinal slots terminating in therecessed skin former surface, to provide a skin former reservoir betweenthe mask and the die, wherein the inward facing longitudinal wall of themask is radially spaced away from the tops of the pins to form a facegap F that provides a terminal opening for the skin former reservoirproximate the tops of the pins, and wherein the mask longitudinallymasks the slots terminating at the skin former surface at a locationimmediately adjacent the die where at least one of the longitudinalslots formed by one or more boundary pins and the corresponding one ormore immediately adjacent interior pins is disposed substantiallyperpendicularly to the inward facing longitudinal wall.

In some embodiments, at least one of the boundary pins comprises a topsurface and a recessed surface, the top surface being generallyco-planar with the discharge surface.

In some embodiments, the recessed surface is a ramp portion of the die.

In some embodiments, all slots in the recessed skin former surface atthe 90's and immediately adjacent to a corresponding boundary pin islocated at a distance equivalent to one half the thickness of theboundary pin, or more, away from the plane of the inward facinglongitudinal wall of the mask.

In some embodiments, a majority of the interconnected matrix oflongitudinal slots are X slots and Y slots, wherein the X and Y slotsare disposed perpendicularly to each other.

In some embodiments, a majority of the slots intersect at right angles.

In some embodiments, the mask longitudinally masks the slots terminatingat the skin former surface at a location immediately adjacent the die atthe 90's of the slot pattern.

In another aspect, a method of forming a die adapted to improve skinuniformity of extruded cellular ceramic substrates is disclosed herein,comprising: providing a slotted block of die material including centralslots adapted to form a cellular matrix of the substrate and peripheralslots located outwardly of the central slots designed to be covered by askin former mask and adapted to extrude peripheral batch material;selecting a target shrinkage; and cutting an arcuate skin formercorresponding to said target shrinkage so as to intersect said slottedblock such that skin flow from tangent slots at 90 degree positions ofthe die is limited to said peripheral batch material. In someembodiments, said cutting of said skin former produces a circular oroval ramp intersecting a discharge face at a front of the die, and aperipheral die surface located outwardly of said ramp; in some of theseembodiments, said skin former ramp forms a partial pin having a slopedpin face at the 90 degree positions of the die. In some embodiments,said plurality of slots are configured to form square cells in thecellular substrate. In some embodiments, the method further comprisesdetermining values for slot width (W), slot spacing (S) and face gap(F), and obtaining ranges of O and I as follows:

Omin=a minimum outer slot to mask distance=½W;

Imax=a maximum inner slot to pin face distance=S−F−Omin;

Imin=a minimum inner slot to pin face distance=½W; and

Omax=a maximum outer slot to mask distance=S−F−Imin,

wherein said skin former is cut at values of O and I in said ranges.

The method can further comprise: calculating first values for O and I,using a first shrinkage, outer dimension of fired substrate, mask radiusand pin face radius; and determining whether said first values for O andI satisfy said ranges for O and I; in some embodiments, if said firstvalues for O and I fall within said ranges for O and I then cutting saidskin former at said first values of O and I corresponding to said firsttarget shrinkage. In some embodiments, if said first values for O and Ido not fall within said ranges for O and I then selecting second valuesfor O and I in said range, and cutting said skin former at said secondvalues for O and I corresponding to said second target shrinkage.

In some embodiments, a difference between a skin thickness correspondingto the 90 degree positions of the die and a skin thickness correspondingto 45 degree positions of the die is not more than 3.00 mils.

In some embodiments, average thickness of skin produced from said die isapproximately 15 mils or less after firing.

The method can be applied to new product designs incorporating alternatecell geometries, dies, and compositions to optimize the product design.

In some embodiments, a method is disclosed herein for forming a die 10adapted to improve skin uniformity of extruded ceramic honeycombsubstrates (a portion of a substrate 11 being shown in FIG. 6). The dieforms a part of a die assembly 12 that includes die 10 and a skin formermask 14 (FIGS. 2 and 4). The die assembly is adapted to extrude ceramicsubstrates each including a matrix of a plurality of cells 16 and a skin18 that is connected to the cell matrix at the periphery of thesubstrate (FIG. 6). The cells of the substrates are defined byinterconnected porous cell walls or webs 20 of the matrix, and somecells are also defined by skin 18, at least in part, and the walls andskin are formed by extrusion. The ceramic substrate is formed as anextruded column having inlet and outlet ends (only one end being shownin FIG. 6). The cells of a ceramic honeycomb substrate can be any shape,e.g. rectangular or square as shown in the figures. In the interiorportion of the ceramic substrate 11, each cell or channel is formed offour web portions 20 a, 20 b, 20 c and 20 d that extend along the lengthof the substrate between its inlet and outlet ends. The die includes aninterconnected matrix of slots 22 defined by a plurality of pins 24.During extrusion plasticized batch material is forced through centralslots 22 a to form the cellular honeycomb matrix, and peripheral slots22 b are covered by the mask 14 (FIG. 2) to form skin layer 18. At theupstream or back end 26 of the die in communication with the extruderare inlet feed holes aligned and communicating with intersecting slots,for example, at alternating intersection points. It should beappreciated that features of the dies shown here are not necessarily toscale or at exact relative dimensions and that for clarity the inletholes and extruder are not shown. Similarly, dimensions, cell densityand shapes of the substrates and die are merely representative and willvary with particular substrates and dies.

Referring to FIG. 2, the skin former mask 14 is a generally annularcollar that includes a downstream or front surface 28, an inward facingsurface 30 forming a central opening 32 and an upstream or back surface34. The mask inward facing surface 30 extends parallel to an extrusiondirection D in which the slots extend. The back mask surface 34 may havedifferent configurations without affecting the present invention but inthis example is shown extending in a radial direction or perpendicularto the inner mask surface 30. The radial direction R is taken radiallyoutwardly from a center of the die perpendicular to the extrusiondirection or slot direction D. The mask 14 is spaced from the die 10 inthe slot direction D such as by using shims (not shown).

The slots 22 can be formed by wire cutting an array of holes into ablock of rigid material such as stainless steel by an EDMelectrochemical machining process. In the case of the square pins shownin FIG. 1, parallel rows X of slots 22 extend in one direction andparallel columns Y of slots 22 extend perpendicular to the row slots(e.g., corresponding to the x and y dimensions of Cartesiancoordinates). A skin former surface portion 36 (FIGS. 1-4) is cut intoan outer periphery of the downstream end surface of the die including anouter peripheral slotted portion, e.g. by using the EDM electrochemicalmachining process. The downstream or front face of the die 10 and theskin former mask 14 define a cavity 38 around a perimeter of the die 10that assists in forming skin which is fed directly by one of the slots22. Referring to FIGS. 3-5, the skin former portion 36 includes a rampsurface 40 cut into pins 24 and extending at an angle from a dischargeface 42 of the die, outwardly and back in an upstream direction relativeto flow of batch material from the extruder, to an outer annularperipheral die surface 44. This peripheral die surface 44 can be ofvarious shapes in this example it is annular and flat and perpendicularto the slot direction D. Skin former surface 36 can comprise a radius 46formed between the peripheral skin former die surface 36 and rampedsurface 40.

As shown in FIGS. 1, 3 and 4, the 90 degree positions of the die aredisposed at four positions in which nearby inner slots are substantiallyparallel or substantially perpendicular to a local tangent to the skinformer boundary, e.g. skin former ramp 40, while 45 degree positions ofthe die are disposed at four positions in which nearby inner slots aredisposed at about 45° with respect to a local tangent to the skin formerboundary for example as seen in FIG. 1 where the 45 degree positionsbisect adjacent 90 degree positions (labeled 90°, 45° in FIG. 1). The 90degree positions of the die and the 45 degree positions of the die arereferred to herein as being at the 90's and at the 45's, respectively.

In general, the cutting of the skin former surface 36 into the slotteddie block results in the formation of partial pins 48 at the skin formerboundary at the 90's (FIGS. 2, 4, 5, 7) at least some of which have adischarge surface 50 on the discharge face of the die and include theramp or pin face 40. A partial pin is defined herein as anything lessthan a full pin after cutting in order to form skin former surface 36.The walls of each full pin are formed by the intersection of twoadjacent row slots and two adjacent column slots. As disclosed herein,the pin face 40 extends from the discharge surface 50 at a top pin faceperiphery 52 of the discharge face (FIG. 4). A face gap F is thedistance between the mask inner surface 30 and the top pin faceperiphery 52 on the discharge face of the die. The width of the face gapF determines the initial extruded thickness of the skin layer, althoughthe final skin thickness may be slightly different than the width ofthat gap due to shrinkage or other factors.

Inner slots are the first slots at the 90's (i.e., the cross-sectionalview of FIG. 4) that are disposed inwardly of a partial pin along thepin face, i.e. defined by the partial pin and the next inwardly disposedfull pin. Outwardly means further from the center of the die (CL) in theradial direction R while inwardly means closer to the center of the dieCL in the opposite direction. If there is more than one partial pin, theinner slot at the 90's is defined as the first slot that is inside theinnermost partial pin. Outer slots are the first slots at the 90's thatare disposed outwardly of the inner slots. As shown in FIGS. 3 and 4,the inner and outer slots 58, 60, respectively, being at the 90's, areparallel to an imaginary line 62 drawn tangent to the pin face periphery52 at the 90's. The inner and outer slots and those slots parallel tothem are referred to herein as tangent slots. For example, the tangentslots at the 90 degree location depicted in FIG. 3 are column slots Y(see FIG. 1). The row slots X in this region extend transverse to thetangent column slots.

Referring to FIGS. 4, 7 and 9 to illustrate distances relevant to theinventive method, slot width (W) is the distance between the tops ofadjacent pins at the discharge face after machining and deposit ofwear-resistant metal on the pin walls in a conventional manner. Slotspacing (S) is a distance between centers of adjacent slots. The pinface diameter or pin face distance is the distance along the dischargeface between the two points at opposing 90's at the pin face peripheryat which the skin former ramp intersects the discharge face (i.e., 2times the distance 64 shown in FIG. 2). At the pin face periphery 52 theramp 40 of the innermost partial pin 48, 48 a intersects the dischargeportion 50 of the die discharge face 42. The die shown in FIG. 9 has twopartial pins 48 a, 48 b and a discontinuous ramp having surfaces 40 a,40 b. Mask diameter 66 is the diameter of the central hole of the mask.As already discussed, the face gap (F) is the distance between the maskinner surface 30 and the pin face periphery 52 along the discharge face42. Outer slot to mask (O) is the distance between the mask innersurface and a center of the outer slot in a direction parallel to thedirection R. Inner slot to pin face (I) is the distance between a centerof the inner slot 58 and the pin face periphery 52 along the dischargeface.

The cut of the skin former portion into the slotted die block ensuresthat when the mask is positioned so as to form the face gap, skin flowamong tangent slots at the 90's is limited to the peripheral batchmaterial 21 fed from peripheral slots 22 b (i.e., the outer slots only).This means that no inner or outer slots feed directly into the face gapF when the die is viewed at the 90's (FIG. 4). If an inner or outer slotfeeds directly into the skin former portion 36 it is not within a regionbetween an imaginary line 51 extending along mask inner surface 30 andan imaginary line 53 extending along pin face periphery 52, the linesextending in the direction D (FIG. 4). A tangent slot feeding directlyinto the face gap at the 90's is not considered herein to feedperipheral batch material or to be a peripheral slot.

The peripheral die surface 44 is spaced back (i.e. upstream) from thedischarge face 42 so as to receive the mask 14 in the cavity 38. A skinformer reservoir 54 is formed between the mask inner and back surfaces30, 34 and the skin former portion 36 of the die (FIG. 4). Not all ofthe peripheral slots 22 b need be active depending upon the design ofthe die, including shim position, but batch material 56 from the activetangent, peripheral slots (e.g., including the outer slots) feeds intothe skin former reservoir 54 and not directly into the face gap at the90's.

The dies of FIGS. 4, 7 and 9 were made based on their compatibility withceramic forming batch material exhibiting 4%, 4.4% and 5% shrinkages(i.e. target shrinkages), respectively that would occur when a ceramicforming material is fired to form the ceramic. These drawings show theskin fanner portion at the 90's. For example, FIG. 4 was taken alongplane 4-4 in FIG. 3 at one of the 90 degree positions of the die. Insome embodiments, all of these dies have the same face gap, slot widthand slot spacing and are designed to produce substrates having the samefired outer contour, density and web thickness within acceptabletolerances. In the die assembly of FIG. 4 as disclosed herein, none ofthe tangent slots at the 90's feeds directly into the face gap (i.e., notangent slot falls in the region between lines 51 and 53). The innerslot 58 only feeds matrix batch material and the outer slot 60 onlyfeeds peripheral batch material 21 into the reservoir behind the mask.The batch from tangent slots that contributes to the skin is thuslimited to peripheral batch material, i.e. batch material that has aradical flow component that supplies the face gap F.

The 4.4% shrinkage target die of FIG. 7 does not satisfy the features ofthe invention in that the outer slot 60 feeds directly into the face gap(i.e., the outer slot 60 is intersected by line 51). Therefore, the skinformer portion does not control skin flow at the 90's for the die ofFIG. 7 because the batch material from the outer slot feeds directlyinto the face gap, which results in nonuniform skin thickness at the90's compared to at the 45's. The die construction of FIG. 9 also is notin accordance with the invention because the outer slot 60 feedsdirectly into the face gap (i.e., is in the region between lines 51 and53), contributing to nonuniform skin thickness at the 90's compared toat the 45's. In addition, this design is disadvantageous because theoutermost partial pins 40 b of the discontinuous skin forming ramp isonly a sliver, which is a potentially weak part of that die.

Ceramic batch material under pressure travels from an extruder to thedie fastened at the outlet of the extruder. The extruder is any typeknown to those skilled in the art such as twin screw or ram type ofextruder. The batch material enters the inlet holes of the die andtravels into both the central and peripheral slots. Most of the batchmaterial travels into the central slots forming the cells of the matrixwhile other portions of the batch material travel into the activeperipheral slots into the skin former reservoir, along the skin formerand mask and to the face gap. The batch material extruded from the facegap forms the skin, which is knitted together with the batch materialextruded from the outermost central slots. Most of the central slotsform full interior cells in the interior of the matrix while theoutermost central slots intersect the skin, forming partial cells justinside the inner dimension of the skin.

The extruded green substrate can then processed in a known manner toproduce a ceramic honeycomb substrate product, including flow throughcatalysts and particulate filters (e.g., diesel particulate filters).The green substrate is formed by cutting the batch material extrudedfrom the die at a predetermined length. The green substrate is in theform of a column having, for example, an oval or circular cross-sectionwith a skin having a thickness t between the outer substrate contour S1to an inner contour S2 (FIG. 6). The inner skin contour isinterconnected with the interior cellular honeycomb matrix. The greensubstrate is fired to produce a ceramic substrate. The substrate may besubjected to some or all of the steps of applying a washcoat andcatalyst material along the porous walls of the cellular matrix,plugging, wrapping with a mat, and canning. Exhaust gas from internalcombustion engines having particulates can be filtered and/or iscatalytically reacted while traveling through optionally pluggedsubstrates. The ceramic honeycomb substrates can thus be made to finalcontour dimensions (i.e., the outer diameter of a fired substrate), celldensity and web thickness.

Extrusion dies can be designed to match the shrinkage of the ceramicbatch composition from which the substrate will be formed. Small changesin shrinkage can be managed in the manufacturing process to obtain afinal exact contour in the fired product. It is thus possible to designdies to match particular shrinkage ranges thereby facilitating greatercontrol of skin uniformity of the honeycomb structure than might be thecase at other particular ranges of shrinkage. Thus, greater control maybe gained by choosing shrinkages and corresponding die parameters thatmaintain pin integrity and control slot location at 90 degree positionsof the die.

A slotted block of die material (e.g., a die block of FIG. 1 beforecutting of the skin former portion), can be provided and a targetshrinkage identified and then the arcuate skin former portion can be cutcorresponding to the target shrinkage by determining values for slotwidth (W), slot spacing (S) and face gap (F), and then using values forO and I in ranges for O and I obtained from the following Equations I:

Omin=a minimum outer slot to mask distance=½W;

Imax=a maximum inner slot to pin face distance=S−F−Omin;

Imin=a minimum inner slot to pin face distance=½W; and

Omax=a maximum outer slot to mask distance=S−F−Imin

Equations I help to ensure that partial pins have adequate integrity andthat skin flow among tangent slots at the 90's is supplied fromperipheral batch material from the outer slots primarily and preferablyonly from the outer slots. Batch shrinkage can be adjusted to achievethe target shrinkage.

In another aspect of the invention first values for O and I arecalculated using a first target shrinkage, outer substrate dimension,mask radius and pin face radius according to Equations II.

Mask Diameter=Fired Contour/(1−% Shrinkage);

Mask Radius=½Mask Diameter;

Pin Face Diameter=Mask Diameter−(2×Face Gap);

Pin Face Radius=½Pin Face Diameter;

Radial Slots=Pin Face Radius/Slot Spacing;

N=whole number of radial slots;

Inner Slot=Distance from die center to inner slot=N×Slot Spacing;

Outer Slot=Distance from die center to outer slot=(N+1)×Slot Spacing;

I=Inner Slot to Pin Face=Pin Face Radius−Inner Slot; and

O=Outer Slot to Mask=Outer Slot−Mask Radius.

Preferably, the ratio I:O is between 1.5:1 to 2.5 to 1, more preferablyapproximately 2:1. Then it is determined whether the first values for Oand I satisfy the target ranges for O and I. If the first values for Oand I satisfy the ranges and are between 1.5:1 to 2.5 to 1 orapproximately 2:1 for O and I, then the die is constructed with thefirst values of O and I and first target shrinkage. If, as is morelikely, the first values for O and I do not satisfy the ranges for O andI or are not approximately 2:1, then a second target shrinkage andcorresponding second values for O and I in the range are selected, andthe die is constructed using the second values for O and I and secondtarget shrinkage. Batch shrinkage for extrusion through this die can beadjusted to achieve the selected second target shrinkage.

EXAMPLE 1

The following die features were selected: a face gap of 0.020 inch, aslot spacing of 0.053 inches, outer dimension of the fired substrate of4.662 inches and a slot width of 0.006 inches. Ceramic-forming batchmaterial having a shrinkage ratio of about 5% could produce a ceramichoneycomb product, such as sold by Corning Incorporated, having adensity of 400 cells/in³ and a web thickness of 3 mils, withinacceptable tolerances.

First values for O and I can be calculated using Equations II asfollows:

Mask diameter=4.662 inches/(1−0.05)=4.907 inches;

Mask radius=2.453 inches;

Pin face diameter=mask diameter−2F=4.907 inches−2(0.02 inches)=4.867inches;

Pin face radius=2.434 inches;

Radial slots=pin face radius/slot spacing=2.434 inches/0.053inches=45.92

Inner slot=45×slot spacing=2.385 inches;

Outer slot=46×slot spacing=2.438 inches;

I=pin face radius-inner slot=2.434 inches−2.385 inches=0.049 inches;

O=outer slot-mask radius=2.438 inches−2.453 inches=−0.015 inches.

Acceptable values for O and I can then be obtained for variousshrinkages using the values for slot width (W), slot spacing (S) andface gap (F) discussed above. The results are shown in the followingTable 1.

TABLE 1 Shrinkage PinFaceRadius-InnerSlot OuterSlot-MaskRadius 5.00%0.049 −0.016 4.75% 0.042 −0.009 4.50% 0.036 −0.003 4.40% 0.033 0.0004.30% 0.031 0.002 4.25% 0.029 0.004 4.20% 0.028 0.005 4.15% 0.027 0.0064.10% 0.026 0.007 4.05% 0.024 0.009 4.00% 0.023 0.010 3.95% 0.022 0.0113.90% 0.021 0.012 3.85% 0.019 0.014 3.80% 0.018 0.015 3.75% 0.017 0.0163.70% 0.016 0.017 3.65% 0.014 0.019

Values for I and O and corresponding shrinkages advantageously fallingwithin a target shrinkage window (satisfying Omin, Imax, Imin and Omax)include shrinkage from 3.65% to 4.30% in Table 1. For example,acceptable values for O (0.010 inch) and I (0.023 inch) correspond to asecond shrinkage value of 4.00% as founding in Table 1. Other values forI and O and corresponding shrinkages, including those for the first 5%shrinkage die of FIG. 9 and the 4.4% shrinkage die of FIG. 7, falloutside of an acceptable range (i.e., do not satisfy Omin, Imax, Iminand Omax values). The 4.4% shrinkage die of FIG. 7 and the 5% shrinkagedie of FIG. 9, which have slots that feed directly into the face gap atthe 90's, are identified by Imin, Imax, Omin and Omax as beingundesirable.

An extrusion die A was constructed corresponding to the 5% targetshrinkage, its O and I values shown in Table 1, and other inputparameters discussed above. An extrusion die B was constructedcorresponding to the 4% target shrinkage, its corresponding values for Oand I of the target shrinkage window shown in Table 1, and other inputparameters discussed above.

FIGS. 3-5 show extrusion die B and FIG. 6 is a ceramic honeycombsubstrate extruded from extrusion die B. FIGS. 8-10 show die A and FIG.11 is a ceramic honeycomb substrate extruded from die A. The samecordierite-forming batch material was used for the extrusion in bothdies except for a batch composition adjustment that produced 4% and 5%shrinkages suitable for the particular die. The cut of the skin formerinto the slotted die block according to the values of O and I thatsatisfy the Equations I, results in substrates having outermost cells atand in the vicinity of the 90's (as shown by brackets in FIG. 6) alongwhich the skin has a uniform thickness. However, the Comparative die Ahaving a design in which the outer slot feeds directly into the face gapat the 90's, had O and I values outside the acceptable target range.This resulted in skin of fired substrates that was relatively thick atthe 90's compared to at the 45's. This skin around outermost slots at orin the vicinity of the 90's is shown by brackets in FIG. 11. Smallpartial pins 40 b were also formed at the 90's in this Comparative die Adesign as seen in FIG. 9, which were undesirably weak.

The average skin thickness and A skin thickness (the skin thickness atthe 90's minus the skin thickness at the 45's) were measured from firedceramic honeycomb substrates produced using dies A1, A2 and B. Theresults are shown in the following Table 2.

TABLE 2 Product Avg. Skin Δ Skin Diameter Thickness 90's 45's thicknessDie (Inches) Contour (mils) (mils) (mils) (units) A1 4.662 5.00% 18.7024.19 12.59 11.60 A1 4.662 5.00% 18.49 23.06 13.92 9.14 A2* 4.662 5.00%18.00 22.48 13.53 8.94 B 4.662 4.00% 13.88 14.85 11.95 2.90 B 4.662 4.0%14.77 14.27 15.31 −1.04 C 4.662 4.25% 15.80 17.30 14.40 2.90 *Dies A1and A2 were identical. “90's and 45's” were the final skin thicknessesmeasured at the 90's and 45's in mils. “Skin Δ” was 90's minus 45's.

Referring to the data of Table 2, the extrusion die B constructed usingthe method disclosed herein at the 4% contour shown in FIGS. 3-5,produced ceramic substrates having much more uniform skin thickness thanthe Comparative 5% extrusion dies A1, A2 having the contour of FIGS.8-10. A similar advantage was also observed in a third die that wasdesigned to 4.25% shrinkage which is also included within the desiredranges for O and I. The fired substrates produced by dies B and Cexhibited a Skin A of not more than 3.00 mils, which was much lower thanthe Skin A for the substrates made from the Comparative dies A1, A2.This uniform skin thickness was achieved for substrates having thinskin, for example, substrates that exhibit an average skin thickness ofapproximately 15 mils.

Based on this information, for example, rather than commerciallyproducing substrates using the 5% shrinkage die, the 5% shrinkage die ofFIGS. 8-10 could be recut to produce the 4% or 4.25% shrinkage die ofFIGS. 3-5. The 5% batch shrinkage material could then be adjusted toproduce matching batch shrinkage material suitable for use with the diesto produce substrates having enhanced skin uniformity in accordance withthe invention.

TABLE 3 Inputs CPSI 400 350 600 600 900 Shrinkage 4.00% 5.00% 3.45%10.40% 11.00% Fired In 4.662 4.662 4.662 4.662 4.662 Diameter Web Mil3.55 5.50 4.50 2.65 2.65 Thickness Face Gap In 0.020 0.020 0.020 0.0160.016 Slot In 0.053 0.056 0.0425 0.045 0.0375 Spacing Die Front SlotWidth Mil 3.70 5.79 4.66 2.96 2.98 Top of In 4.81622 4.86737 4.788595.17113 5.20620 Pin ID Pin Face 2.40811 2.43368 2.39429 2.58556 2.60310Radius Mask ID In 4.856 4.907 4.829 5.203 5.238 Inner Tang In 0.0230.024 0.014 0.021 0.016 Slot to Pin Face Diam (I) Outer Tang 0.010 0.0120.008 0.008 0.006 Slot from Mask (O) I:O 2.3 2.0 1.75 2.63 2.65 Inner0.031 0.035 0.019 0.026 0.019 Max Outer 0.002 0.003 0.003 0.003 0.002Min Inner 0.003 0.004 0.003 0.002 0.002 Min Outer 0.030 0.034 0.0190.026 0.019 Max

EXAMPLE 2

The following input parameters produced acceptable Inner Tang Slot toPin Face Diameter (“I) and Outer Tang Slot from Mask (“O”) values in thefollowing Table 3.

While any of the inputs can be changed to accommodate new productspecifications, in commercial production the slot spacing and slot widthwould be fixed because changing them would require new die fabrication.Face gap could be adjusted in a known manner by using a mask having adifferent mask diameter.

By way of example, the 5% shrinkage die of Table 3 can be fabricatedusing the above input parameters for making a product having a 4.662inch fired outer diameter. If a fired contour of 4.162 inches isrequired, a changed shrinkage, e.g. 4.3%, can be entered and a smallerpin face diameter would produce acceptable O and I values. One or moreother input parameters could also be varied to see whether theacceptable O and I window is obtained. The 5% shrinkage die could bere-cut to a 4.3% shrinkage die having a smaller pin face diameter. Thiswould then require adjusting batch shrinkage to 4.3%.

Many modifications and variations of the invention will be apparent tothose of ordinary skill in the art in light of the foregoing disclosure.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention can be practiced otherwise than has beenspecifically shown and described.

What is claimed is:
 1. A porous ceramic honeycomb body, comprising: ahoneycomb structure comprising perpendicularly intersecting porousceramic walls; and a coextruded skin surrounding and directly adjacentto the honeycomb structure, the skin comprising first portions ofuniform thickness t with each other at the 90 degree positions of thehoneycomb structure and second positions of a thickness greater than theuniform thickness t off the 90 degree positions of the honeycombstructure.
 2. The porous ceramic honeycomb body of claim 1, wherein adifference between the first portions of uniform thickness t and thesecond positions of a thickness greater than t is not more than about3.00 mils.
 3. The porous ceramic honeycomb body of claim 2, wherein theaverage thickness of the skin is about 15 mils or less.
 4. The porousceramic honeycomb body of claim 1, wherein the second portions of athickness greater than t are at the 45 degree positions.
 5. The porousceramic honeycomb body of claim 1, wherein the perpendicularlyintersecting porous walls define cells and the first portions of uniformthickness t each extent a plurality of cell widths.
 6. The porousceramic honeycomb body of claim 1, further comprising a circular or ovalcross section.
 7. The porous ceramic honeycomb body of claim 1, whereinthe perpendicularly intersecting porous walls define square orrectangular cells.
 8. The porous ceramic honeycomb body of claim 1,further comprising a washcoat and catalyst material along the porouswalls and/or plugging of cells defined by the perpendicularlyintersecting porous walls.
 9. A method of making a porous ceramichoneycomb body comprising a coextruded skin, the method comprising:extruding ceramic precursor batch through an extrusion die to form agreen substrate, wherein the die comprises: a slotted block of diematerial including central slots adapted to form a cellular matrix ofthe substrate and peripheral slots located outwardly of the centralslots designed to be covered by a skin former mask and adapted toextrude peripheral batch material; an arcuate skin former correspondingto a target shrinkage so as to intersect said slotted block such thatskin flow from tangent slots at 90 degree positions of the die islimited to said peripheral batch material; and wherein values for a slotwidth (W), a slot spacing (S) and a face gap (F), and ranges of O and Ias follows:Omin=a minimum outer slot to mask distance=½W;Imax=a maximum inner slot to pin face distance=S−F−Omin;Imin=a minimum inner slot to pin face distance=½W; andOmax=a maximum outer slot to mask distance=S−F−Imin,wherein said skin former is disposed at values of O and I in saidranges.
 10. The method of claim 9, further comprising firing the greensubstrate to form a porous ceramic honeycomb body, and at least one of:applying a washcoat and catalyst material along porous walls of theporous ceramic honeycomb body, plugging channels of the porous ceramichoneycomb body, and canning the porous ceramic honeycomb body.
 11. Amethod of making a porous ceramic honeycomb body comprising a coextrudedskin, the method comprising: extruding ceramic precursor batch throughan extrusion die to form a green substrate, and firing the greensubstrate to produce a porous ceramic honeycomb body, wherein the porousceramic honeycomb body comprises: a honeycomb structure comprisingperpendicularly intersecting porous ceramic walls; and a coextruded skinsurrounding and directly adjacent to the honeycomb structure, the skincomprising first portions of uniform1 thickness t with each other at the90 degree positions of the honeycomb structure and second positions of athickness greater than the uniform thickness t off the 90 degreepositions of the honeycomb structure.
 12. The method of claim 11,wherein a difference between the first portions of uniform thickness tand the second positions of a thickness greater than t is not more thanabout 3.00 mils.
 13. The method of claim 12, wherein the averagethickness of the skin is about 15 mils or less.
 14. The method of claim11, wherein the second portions of a thickness greater than t are at the45 degree positions.
 15. The method of claim 11, wherein theperpendicularly intersecting porous walls define cells and the firstportions of uniform thickness t each extent a plurality of cell widths.